1. Field of the Invention
This invention relates to a line interface for data access arrangements (DAA). More particularly, it relates to an adaptive line powered codec.
2. Background of Related Art
Many portable computer devices utilize modems and other data devices for communicating over a telephone line. In such devices, battery size and weight is an important consideration. A balance must be reached between the size and weight of the overall device, which is dictated largely by the choice of battery, and an acceptable length of operation between charges.
Unfortunately, although operating an acceptable length of time when running typical application programs, the modems and other data communication devices of a portable computer utilize a large amount of power when communicating over a telephone line. The battery source which powers both the portable computer device and its modem is typically sized for general computing applications, and runs out of power quickly when actively communicating over a telephone line through a modem. Portable computer devices such as personal digital assistants (PDAs), hand held PCs (HPC), PCMCIA modems, and portable data terminals are designed to operate up to several hours on a single battery charge, but operate only fractions of an hour on a single battery charge when communicating via modem. Thus, although portable computer devices operate a sufficient length of time for quick data transfers over a modem powered completely by a battery, they typically require that external AC power be applied to allow for longer uses of the modem. It is therefore desirable for battery powered computer devices including a modem to draw power, in addition to the inherent battery, from a secondary power source.
The DC power inherent in a telephone line provides a convenient source of power, but there are often limitations and restrictions which limit the ability of a modem to derive power from the telephone line. For instance, present regulations in the United States are such that significant current may only be drawn from the telephone line when the telephone or modem is in an off-hook or active condition. In order to hold the telephone line in an off-hook condition, current in the approximate range of 13 milliamps (mA) to 150 mA must be drawn. Thus, the maximum amount of current drawn from the telephone line is limited.
Modems which are designed to be powered entirely from the telephone line are known, but these designs either suffer from an extremely constrained power budget, or are wasteful of the available current. Moreover, modems in general are also subject to government constraints, e.g., FCC Part 68 requirements for telephones in the U.S., and limitations on effects and noise which may be placed back on the telephone line, placing further restrictions on the use of power from the telephone line.
An exemplary line powered modem is described in U.S. application Ser. No. 09/028,061, entitled xe2x80x9cLow Noise Line Powered DAA With Feedbackxe2x80x9d by Hollenbach et al., filed Feb. 23, 1998, the entirety of which is explicitly incorporated herein by reference.
A Data Access Arrangement (DAA) provides the physical interface between a data source such as a modem, and a telephone line. The DAA is responsible for presenting the proper DC termination and AC modulation characteristics to the telephone line. For instance, the DAA must draw a minimum amount of DC current when in the off-hook condition to hold the telephone line in an off-hook condition, but at the same time must draw no more than a maximum amount of current while in the off-hook condition. Thus, the DC termination or load must be within prescribed limits corresponding to the pertinent telephone standards of the country in which the DAA is being used. Accordingly, the modem must operate with no more than the prescribed maximum current available from a telephone line in the particular country in which the DAA is being used.
In today""s global economy, it is desirable to design and manufacture products for use in any of a plurality of different countries, not just for a single country. However, the regulations with respect to the minimum and maximum amount of current the customer premises equipment must draw (i.e., the DC load) to meet the off-hook specifications varies greatly in different countries. Thus, a DAA used in one country must provide characteristics to a telephone line which may be and often are entirely different from the characteristics presented by a DAA used in another country. Accordingly, manufacturers conventionally manufacture different DAAs including different physical components for each of the varying countries, or manufacture a DAA with termination components which are switched in and out based on the requirements of the particular country in which the device is used. Manufacture of separate DAAs for separate countries, and/or the manufacture of DAAs having switchable components for use in various countries, increases the overall costs of providing such DAAs in a global marketplace. Moreover, the reliability may be degraded due to the higher parts count.
There is a need for a cost effective and reliable DAA capable of being powered at least in part by the available line current presented by the various countries.
In accordance with the principles of the present invention, a line powered codec comprises a line powered analog-to-digital converter, and a line powered digital-to-analog converter. A DC current measurement module is adapted to determine an amount of current available for use from a telephone line in an off-hook condition. The analog-to-digital converter and the digital-to-analog converter are each adapted to be maintained in a reset condition when the DC current measurement module determines that insufficient current is available from the telephone line in the off-hook condition.
A method of powering up a line powered codec in accordance with another aspect of the present invention comprises determining an approximate amount of available current at a tip of the line powered codec connected to a telephone line. The line powered components of the line powered codec are allowed to operate if the determined amount of available current is sufficient. Otherwise the line powered components of the line powered codec are retained in a reset state.
A method of maintaining a minimum current draw from a telephone in a line powered codec capable of programmably adjusting an output impedance in accordance with yet another aspect of the present invention comprises determining an approximate amount of available current at a tip of the line powered codec connected to a telephone line. the line powered components of the line powered codec are allowed to operate if the determined amount of available current is sufficient, otherwise a minimum DC voltage level is maintained at the tip of the line powered codec from a processor on a low power side of the line powered codec.